Preheat an outdoor grill for high heat; close cover. Place four 6-inch square pieces of aluminum foil on a work surface.
Prick each potato several times with a fork and place onto a square of foil. Top each potato with 1 tablespoon butter and sprinkle with parsley and garlic salt. Wrap foil around each potato.
Reduce the heat of the grill to medium. Place potatoes on the grate. Grill until tender, turning occasionally to ensure even cooking, about 20 minutes.
Remove from the grill. Unwrap the foil and serve with sour cream.